Composite metal structures of all kinds have great appeal in almost all fields of technology as these structures are capable of providing overall physical property characteristics frequently not found in a given metal or alloy thereof. It is not surprising to find there are uses for metal structures that call for metal surfaces of the structure that require hard surfaces for wear, contact fatigue and bending fatigue, while another portion of the structure has lower service stress requirements but does require toughness and sheer strength to, for example, transmit torque. Gear teeth, splines and bearing races are examples of metal surfaces of the character noted above, whereas lower service stress requirements are found in gear web and gear shaft portions.
Historically, carburized, nitrided, or carbonitrided alloy steels have been used for the manufacture of virtually all aircraft gears and gear shafts. Typically carburized alloy 9310 has been the material of choice.
Driven by an ever present need to continuously reduce the weight of all parts of an aircraft while maintaining structural integrity it is no surprise that the aircraft industry has pressed for the creation of lighter weight high strength gears for use in aircraft environment. It is well recognized that every ounce of weight that can be removed from an aircraft will pay large dividends in the way of fuel savings over the life of the aircraft, which life frequently exceeds twenty years.
To meet the need for a light weight high strength gear as defined above it has been suggested that steel toothed gears with titanium webs and shafts be employed to meet the need. Accordingly it has been recognized that a composite gear with steel teeth and titanium web or shaft would provide the desired weight reduction, however, to date there have been no reliable methods of welding titanium directly to steel on a production basis due to metallurgical incompatibilities.
This is not to say that others have not tried their hand at creating composite structures that have included steel and titanium. In this regard attention is directed to the W. F.Sharp, Jr. U.S. Pat. No. 3,798,011('011) which describes a multilayered metal composite that finds utility in transition joints. The '011 patent identifies a variety of metals that may be fashioned into layers and includes among such metals iron (Fe), aluminum (Al), and titanium (Ti) and further notes that where metals of an interlayer and outer layer together form a ductile alloy, the bond between layers may be prepared for example, by roll bonding, inertia (i.e. friction) welding, flash-but welding, or by explosion bonding.
While the '011 patent makes publicly available the ideas that a composites plate of Fe, Al and Ti is obtainable and that friction welding of ductile metal layers are a way to bond layers together, the '011 patent offers no suggestions relevant to the method of the instant invention which brings into existence a composite multi-metal steel gear having a titanium gear web and shaft.
A method for manufacturing an improved composite gear having a steel hub and bronze gear teeth is shown and described in the R. L. Wolfe et al U.S. Pat. No. 3,557,423('423). The steel hub and a bronze annulus containing the gear teeth are joined by solder.
While the '423 patent teaches the concept of a composite metal gear there is no suggestion of a multi-metal gear/shaft of the nature of the subject invention to be described more fully hereinafter.
Another composite metal gear is described but not shown is the Neal et al U.S. Pat. No. 4,964,564('564). In the "Background of Invention" Section of the '564 patent it is indicated that the invention of the '564 patent may be employed in the manufacture of gears where the teeth are required to be made of titanium or light alloy to reduce the rotating mass. It is noted, however, that no where in the balance of the '564 patent specification and claims is there any further mention of how the invention is incorporated in the manufacture of composite steel/titanium gears. The patent drawings and specification do describe the joining of two bodies together by means of an interlayer of a different material where surfaces of the two bodies to be joined are provided with mating corrugated configurations.
The bonding of steel and titanium directly to each other without the presence of an intermediary layer of material is shown and described in U.S. Pat. No. 3,629,932 ('932) and 5,054,980 ('980). In the '932 patent a titanium plate is driven by an explosive force against a steel plate to create a composite metal sheet. In the '980 patent a titanium plate is secured to a carbon steel plate by means of a titanium stud that includes an internal steel friction weldable nose that bonds to the steel plate through an opening therein. The titanium stud is then friction welded to the titanium plate. The steel to titanium composite methods of the '932 and '980 patents appear to represent the state of the art and do not suggest the novel multi-metal composite structure present in the gear/shaft structure embodying the invention as defined hereinafter.
A process for the inertia welding of steel to aluminum is described in the Marion Calton U.S. Pat. No. No. 3,597,832 ('832). The process of the '832 patent utilizes a conical projection on an aluminum workpiece that is brought into frictional contact at the center of a steel workpiece. As the conical projection is rotated and increased pressure is applied to bring the conical projection against the steel workpiece a friction weld of the aluminum to the steel is accomplished.
In the Hoch et al U.S. Pat. No. 3,693,238 ('238) there is described a method of joining together an aluminum workpiece and a ferrous workpiece by relative rotation of the workpieces while the workpieces are forced together at mutually engaging parallel, planar surfaces.
The '832 and '238 patents described above are believed to represent the state of the art with respect to the friction bonding of aluminum to steel and do not suggest the process of the instant invention which produces a multi-metal composite gear/shaft.